Thanks for your comments,looks like there is more to it than i thought.Fred.Fred
Traditionally, porches are measured in 3 vertical heights, which are 7 ft, 7 ft 6 inches, or 8 ft, (awning rail to ground) but now go by their metric equivalent. Within reason you can slide a porch awning along the awning rail to whichever position suits you best (but not blocking doors, windows or vents). They also come in various widths and depths.
I believe the above also applies to the more modern semi-dome shaped porches.
Hope this is helpful.